The present invention relates to a via connection structure in a RF circuit. More particularly, it relates to a via connection structure with a compensative area on a reference plane to achieve the well impedance-matching for the RF circuit.
The common electronic products comprise of one or more IC chips disposed on a substrate in the recent days. The chips could receive the signals from the substrate or from the outside devices; likewise the signals calculated by the chips transmit to the substrate or to the outside devices. Therefore, the quality of signal transmission in the substrate influences on the accuracy of the transmitting signals received by the chips or the substrate. In other words, the poor quality of the signal transmission will induce the fault or the malfunction of the electronic products.
The quality of the transmitting signal depends on the variation of the characteristic impedance in the signal transmission. Furthermore, if the characteristic impedance of the signal transmitting path is different to that of the system, i.e. the impedance mismatch, it causes a portion of the signal reflected while the signal transmits to the chips or the outside devices. The ideal condition which the characteristic impedance of the signal transmitting path matches to that of the system will cause the maximum propagation in the signals. Otherwise, most of the signal is reflected while the impedance mismatch is serious. On one hand, the chip could not receive the correct signals from the substrate; one the other hand, the substrate could not receive the correct commands from the chips. Thus, it will cause a fault, or a malfunction of the system operation.
The structure of a common substrate comprises a plurality of conductive layers and a plurality of insulation layers alternately stacked. The circuits designed for various functions are formed by the patterned-circuits on several conductive layers. The circuits on the different conductive layers are isolated by the insulation layers between of them. Thus, the via connection structure is used for the interconnection between two or more different conductive layers. The via connection structure comprises at least one via which is filled with the conductive material or is covered with the conductive material on the sidewall to electrical connect two different conductive layers. In general, the characteristic impedance is seriously deviated while the signal passing through a via connection structure. In other words, it is necessary to modify the structure to compensate the impedance deviation for the signal quality.
Please refer to FIG. 1a and FIG. 1b, wherein FIG. 1a is the schematic cross-sectional view of the four-layered substrate according to the prior art, and FIG. 1b is the schematic top view of patterned-circuits in the four-layer substrates. The substrate 100 comprises three insulation layers 112, 114, 116, a power plane 120 and a ground plane 130. The power plane 120 is disposed between the insulation layer 112 and the insulation layer 114; similarly the ground plane 130 is disposed between the insulation layer 114 and the insulation layer 116. The substrate 100 further comprises the first conductive line 142 formed on the insulation layer 112, the second conductive line 144 formed on the insulation layer 116, and a via 102 through those insulation layers 112, 114, 116, the power plane 120, and the ground plane 130. The via connection structure 140 comprises a conducting via 146 which fill in the via 102 or cover the sidewall of the via 102, the via pad 148 formed on the insulation layer 112, and the via pad 150 formed on the insulation layer 116. Two ends of the conducting via 146 electrically respectively connect to the via pad 146 and to the via pad 150. Furthermore, the conducting via 146 electrically couples to the first conductive line 142 through the ring-shaped via pad 146 and to the second conductive line 144 through the ring-shaped via pad 150.
The parasitic capacitance and the parasitic inductance of the via connection structure depend on the dielectric constant of the insulation layer, the thickness of the insulation layer, and the dimensions of the via connection structure. Please refer to FIGS. 2a and 2b respectively illustrated the character responses of the via connection structures of two different diameters in frequency domain and in Smith Chart. The curve 201 in FIG. 2a and the curve 202 in FIG. 2b are illustrated the characteristic response of the via connection structure having one via which the diameter of the via is 8 mil. Likewise, the curve 211 in FIG. 2a and the curve 212 in FIG. 2b are illustrated the characteristic response of the via connection structure having one via which the diameter of the via is 20 mil. The curve 202 in FIG. 2b is in the upper part of Smith Chart. It means that the parasitic inductance dominates the characteristic response of the 8-mil via connection structure. On the other hand, the curve 212 in FIG. 2b is in the lower part of Smith Chart. The parasitic capacitance dominates the characteristic response of the 20-mil via connection structure.
In respect to the parasitic capacitance, the parasitic capacitance between the via connection structure 140 and the power plane 120 and that between the first conductive line 142 and the power plane 120 are different. There are also different in the parasitic capacitance between the via connection structure 140 and the ground plane 130 and in that between the second conductive line 144 and the ground plane 130. Concerning about the parasitic inductance, the parasitic inductance induced by the via connection structure 140 is different from that induced by the first conductive line 142, or is different from that induced by the second conductive line 144. Thus, the difference in the parasitic capacitance and the parasitic inductance will induce the characteristic impedance deviation while the signal passes through a via connection structure, even more the malfunction of the system.